The short version. Pick the right isotope for the check (Cs-137 is the workhorse for constancy), choose an activity with comfortable margin — well above the bare minimum, not just barely usable — and use a source geometry that drops into the well the same way every time. Don’t fixate on the source’s age: a long-lived source decays slowly and predictably, and the counter corrects for that automatically. The reason to retire a source is almost always regulatory, not performance.

1. The right isotope

For a routine constancy or efficiency check, you want an isotope that is long-lived, has a single clean photopeak, and is easy to source. Cs-137 is the standard choice on almost every wipe and well counter: one photopeak at 662 keV, a 30-year half-life so it lasts decades, and broad availability as a sealed source. Co-57 (122 keV) is the common companion for the low-energy end. The point of the check is to confirm the instrument reads a known source the same way today as it did yesterday — so a stable, simple, long-lived isotope is exactly what you want.

2. Enough activity — with margin

This is where good intentions go wrong. A counter measures by collecting counts, and the certainty of every measurement depends on how many counts it can collect. A source with healthy activity delivers a high count rate, so the instrument reaches a statistically solid number quickly and locks cleanly onto the photopeak. A source that is too weak does neither: the count rate is low, every reading carries more statistical noise, and below a certain point the instrument struggles even to find and seat the peak reliably during calibration.

The trap is assuming a low-activity source is “safer” or “cleaner.” It is neither. A source that is an order of magnitude weaker than it should be will pass a casual glance and then quietly make your efficiency checks wander, because the instrument is working at the edge of its counting statistics. As a rule of thumb, choose a Cs-137 check source comfortably above ~100 nCi — a few hundred nanocuries is a sensible, long-lasting choice — and steer well clear of the very low-activity sources sometimes offered for simple presence-testing. They are not built for quantitative QC.

Why “weaker” backfires. Detection efficiency at 662 keV is only a few percent, and a meaningful QC count needs to accumulate tens of thousands of events to be statistically sound. Halve and halve again the source activity and you are no longer collecting enough counts in a reasonable count time — so the check that is supposed to reveal instability instead adds it. More activity, within your licensing, is almost always the better choice.

3. A source that fits — the same way every time

Counting efficiency depends on geometry: where the source sits in the well, and how reproducibly you can put it there. A source whose diameter and length suit the well, and which seats the same way on every insertion, gives you a stable baseline. If you change to a different source — a different size, shape, or activity — expect the absolute efficiency reading to change, because the geometry changed. That is normal; simply re-establish your baseline with the new source, confirm it reads above the instrument’s minimum, and watch for drift from there.

Age is not the enemy — the counter handles decay

Operators often worry that a source several years old is “past it.” For a long-lived isotope, that worry is usually misplaced. Cs-137’s 30-year half-life means a source loses only about 2% of its activity per year; one that is fifteen years old still has roughly 70% of its original strength — plenty for a clean check.

And you do not have to track that decay by hand. On an LTI counter you enter the source’s original activity and its reference date one time, and from then on the instrument decay-corrects to the present moment automatically, every time you run a check. You always enter the original figures; the counter computes today’s true activity for you. There is no hand calculation, and no need to enter a “decayed” value yourself.

When to replace a source

The reason to retire a sealed source is almost always regulatory, not performance: a recommended working life, or a periodic leak/wipe test required by your radiation-safety program. As long as a long-lived source still delivers a healthy count rate — which it will for many years — the instrument is perfectly happy to keep using it. Replace it when your compliance rules call for it, not because the calendar makes you nervous.

What to do

  • Match the isotope to the check — Cs-137 for general constancy, Co-57 for the low-energy point.
  • Buy activity with margin — comfortably above ~100 nCi for Cs-137; don’t shop for the weakest source on the shelf.
  • Keep geometry reproducible — a source that fits the well and seats the same way every time; re-baseline if you switch sources.
  • Enter original activity + reference date once — let the counter handle decay; never hand-decay a value.
  • Replace on compliance, not on age — leak-test and working-life rules drive retirement, not performance.

Further reading